G'day mate
For the most part, you would probably use SMS 1020. This c/f pattern/colour would generally be found on parts of the car where things like structural strength, rigidity, safety are at their highest i.e. driver's cell, floor, wings. For things like brake ducts, engine intake scoops, steering wheels, etc, you could experiment with 1120, 1220 & 1420. For suspension arms I use 1/24 c/f decal because it just looks a little more scale for these smaller pieces.

There are plenty of pics online & more then likely many models on here which should guide you too.